Why are cells exposed to substance X during t=8-12 and not t=8-16 ?

FACS analysis revealed that cells are arrested after R point transition and before G1/S transition. This occurs around t=8-12. The cell cycle components that are active during that time window may differ from those involved in later stages (t=12-16), and so we thought that in order to study possible mechanisms underlying substance X effects, it is better to minimize the duration of exposure to substance X to the time when it is most effective.